A method of imaging using interdigitated electrodes, a photoconductive layer and a magnetic imaging layer

ABSTRACT

A composite recording medium comprises a photoconductive layer, a plurality of interdigitated first and second electrodes in electrical contact with and contiguous to the photoconductive layer, and a magnetic recording layer in heat-transfer relationship with and contiguous to the photoconductive layer. Methods and apparatus for magnetically recording an image with the assistance of this composite recording medium are also disclosed.

United States Patent 1191 Duck et a1.

[54] METHOD OF IMAGING USING INTERDIGITATED ELECTRODES, APHOTOCONDUCTIVE LAYER AND A MAGNETIC IMAGING LAYER [75] Inventors:Sherman W. Duck, Alhambra;

Frederick J. Jeffers, Altadena; James U. Lemke, Del Mar, all of Calif.

[73] Assignee: Bell & Howell Company, Chicago,

Ill.

[22] Filed: April 17, 1970 21 Appl. Nb; 29,584

[52] US. Cl, ..96/l R, 96/1.5, 96/1 E, 346/74 M, 346/74 MT, 117/211,117/212, 117/215,117/217,l17/21,8

' s11 lnt.Cl. ..o03 13/14,o03 13/22 [58] Field of Search ..96/11.5, 1, lE; 346/74 MT 51 Feb. 20, 1973 [56] References Cited Y UNITED STATESPATENTS 3,485,621 12/1969 Kazan ..96/1

3,288,602 1 1/1966 Snelling et a1. ..96/l 2,836,766 5/1958 Halstead96/1.5 X 2,798,959 7/1957 Moucrieff-Yeates.... ..96/l E PrimaryExaminer-George F. Lesmes Assistant Examiner-Roland E. Martin, Jr.Attorney-Luc P. Benoit [57] ABSTRACT A composite recording mediumcomprises a photoconductive layer, a plurality of interdigitated firstand second electrodes in electrical contact with and contiguous to thephotoconductive layer, and-a magnetic recording layer in heat-transferrelationship with and contiguous to the photoconductive layer. Methodsand apparatus for magnetically recording an image with the assistance ofthis composite recording medium are also disclosed.

15 Claims, 3 Drawing Figures I PATfiNTEflr azbl'sis I IN VENTORS SHERMANw. DUCK FREDERICK J. JEFFERS JAMES u. LEMKE Arne v5)",

METHOD OF IMAGING USING INTERDIGITATED ELECTRODES, A PI-IOTOCONDUCTIVELAYER AND A MAGNETIC IMAGING LAYER BACKGROUND OF THE INVENTION 1. Fieldof the Invention The subject invention relates to magnetic recordingand, more particularly, to the magnetic recording of images with theassistance of thermal gradients.

2. Description of the Prior Art Magnetic imaging has been the subject ofserious investigation in. recent years, since it has several advantagesover more conventional imaging techniques.

For instance, magnetic imaging offers the prospect of an avoidance oftim-consuming and delicate chemical processing steps now required incustomary photography. Magnetic imaging also ofiers the prospect of anavoidance of expensive and potentially dangerous highvoltage equipmentnow required in electrostatic xerography and related techniques.

Unfortunately, magnetic imaging techniques which have become publicallyknown to date cannot compete in terms of light sensitivity and exposurespeed with photographic methods or even with electrostatic xerography.

SUMMARY OF INVENTION The subject invention overcomes or materiallyalleviates the above mentioned disadvantages and, from one aspectthereof, resides in a method of magnetically recording an image,comprising in combination the steps of providing a plurality ofinterdigitated first and second electrodes, providing a photoconductivelayer in electrical contact with and contiguous to the interdigitatedfirst and second electrodes, and providing in heat-transfer relationshipwith and contiguous to said photoconductive layer a magnetic imagerecording layer susceptible to an imagewise change of magnetization inresponse to a thermal image pattern. This method further includes thesteps of connecting said interdigitated first and second electrodes toopposite terminals, respectively, of a source of electric energy andapplying electrical energy from said source to the interdigitated firstand second electrodes by way of said opposite terminals and exposing thephotoconductive layer to the image to provide a thermal patterncorresponding to theimage and constituting the thermal image pattern,and magnetically recording the image by changing the state ofmagnetization of the recording layer in response to the thermal pattern.

By way of example, the interdigitated first and second electrodes may belocated between the photoconductive layer and the magnetic recordinglayer. This arrangement has the advantage that the electrode structuredoes not obscure the image-exposed top surface of the photoconductivelayer, and

that heat imparted to the electrodes is readily transferred to themagnetic recording layer.

Alternatively, at least part of the photoconductive layer may be locatedbetween the'interdigitated first and second electrodes and the magneticrecording layer. A preferred embodiment of the invention in accordancewith this alternative solution has the photoconductive layer providedwith a first side located at the. plurality of interdigitated first andsecond electrodes, and with a second side opposite such first side. Themagnetic recording layer is then provided in heat-transfer relationshipwith the photoconductive layer through the latter second side of thephotoconductive layer.

The latter alternative solution is presently preferred because availabletechniques favor the provision of an electrode structure on a substrateover the provision of an electrode structure on a photoconductive layer,and because problems of light absorption by the photoconductive layerare alleviated if the electrodes are at the image-exposed side of thephoto-conductive layer.

From another aspect thereof, the subject invention resides in a methodof magnetically recording an image on a composite recording medium,which medium includes a photoconductive layer, a plurality of inter- Idigitated first and second electrodes in'electrical contact with andcontiguous to the photoconductive layer, and, contiguous to thephotoconductive layer a magnetic image recording layer susceptible to animagewise change of magnetization in response to a thermal imagepattern. This method comprises in combination the steps of applyingelectrical energy from said source to the interdigitated first andsecond electrodes by way of said opposite terminals and progressivelyexposing the photoconductive layer to the image in a directionprogressing parallel to the interdigitated electrodes to produce athermal pattern corresponding to the image and constituting said thermalimage pattern, and mag- I netically recording said image by changing thestate of magnetization of the recording layer in response to the thermalpattern.

The subject invention also resides in apparatus for magneticallyrecording an image, comprising in combination a plurality ofinterdigitated first and second electrodes, a photoconductive layer inelectrical contact with the interdigitated first and second electrodes,and a magnetic recording layer in heat-transfer relationship with thephotoconductive layer. This apparatus further includes means connectedto the first and second electrodes for electrically energizing thephotoconductive layer, means for exposing the electrically energizedphotoconductive layer to the image to produce a thermal patterncorresponding to the image, and means operatively associated with saidmagnetic recording layer for changing the state of magnetization of therecording layer in response to the thermal pat- I tern whereby amagnetic record of the image is established.

As already indicated above in connection with a method aspect of thesubject invention, theinterdigitated first and second electrodes may,for example, be located between the photoconductive layer and themagnetic recording layer or, alternatively, at least part of thephotoconductive layer may be located between The subject inventionfurther resides in a composite recording medium operable with a sourceof electric energy having opposite terminals, comprising in combinationa plurality of interdigitated first and second electrodes means forconnecting said interdigitated first and second electrodes to oppositeterminals, respectively, of said source of electric energy, aphotoconductive layer in electrical contact with and contiguous to theinterdigitated first and second electrodes, and a thermally responsivemagnetic image recording layer in heat-transfer relationship with andcontiguous to the photoconductive layer.

In accordance with a presently preferred embodiment of the subjectinvention, the photoconductive layer mentioned in the precedingparagraph has a first side located at the named plurality ofinterdigitated first and second electrodes, and has a second sideopposite such first side. The magnetic recording layer is in thisembodiment in heat-transfer relationship with the photoconductive layerthrough the latter second side of the photoconductive layer.

The above mentioned combination of photoconductive layer, interdigitatedelectrodes, and magnetic recording medium constitutes an importantfeature of the subject invention, either by itself or in combinationwith other features.

To be sure, the use of interdigitated electrodes on photoconductors isold in the art of photoelectric devices. In the field ofelectrophotography, however, to a subgroup of which the subjectinvention belongs, the use of interdigitated electrodes on one side ofthe photoconductive layer has so far been generally rejected, andemphasis has been placed on the use of sandwich configurations in whichthephotoconductive layer is located between a pair of sheet-likeelectrodes.

In the course of intensive investigations into magnetic imaging we have,however, been able to reach the conclusion that the prior-art bias isunfounded and that an interdigitated electrode approach presents,indeed, the best overall solution as far as the particular field ofmagnetic imaging is concerned.

In particular, we have found that the interdigitated electrode structureacts in the manner of a halftone plate which materially improves imageresolution and quality. We have also found that the interdigitatedelectrode structure greatly facilitates the premagnetization of themagnetic recording layer, which is important where information isrecorded by a selective demagnetization of premagnetized material.

We have also found in the course of extensive tests that compositemagnetic recording media of the type herein disclosed withinterdigitated electrode structures display less sensitivity to pinholes and lumps in the photoconductor. In sandwich-type photoconductivemedia, these-irregularities lead to very annoying ef-, fects in theprinted-out image.

We have further come to the conclusion that the heavy electric currentsrequired for the thermomagnetic imaging effect require that thetransparent top electrode in sandwich-type arrangements has to have aconsiderable thickness, which can lead to severe transmission losses inthe incoming image. In contrast thereto, our interdigitated electrodestructure does not impair light transmission in the areas between theconductors. This feature alone permits an increase in sensitivity by afactor of two or more, despite the fact that sity required for theoperation of the recording medium is accomplished, and a superiorrheology is still preserved, since every light-exposed point is servicedby two adjacent electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readilyapparent from the following detailed description of preferredembodiments thereof, illustrated by way of example in the accompanyingdrawings, in which:

FIG. 1 is a perspective view of an imaging apparatus in accordance witha preferred embodiment of the subject invention;

FIG. 2 illustrates a first phase in the operation of the apparatus ofFIG. 1; and

FIG. 3 illustrates further phases in the operation of the apparatus ofFIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS The apparatus 10 of FIG. 1 includesa composite photoelectromagnetic recording medium 12 which embodies-keyfeatures of the subject invention. The composite recording medium 12includes a transparent substrate 13 which may be a sheet of glass or anorganic equivalent thereof, such as a transparent high-temperaturepolyimide or polybenzamidazole.

An interdigitated electrode structure 14 including interdigitated firstand second electrodes 15 and 16 is preferably located on the transparentsubstrate 13, although the electrodes may alternatively be located onthe photoconductive layer presently to be described. The deposition ofelectrode structures on a transparent substrate is a well-establishedart and does not as such form part of the subject invention. Suffice tosay therefore, that the electrode structure 14 may be deposited on thesubstrate 13 by evaporation, painting or sputtering, and that preferredelectrode materials include gold, indium, chromium, and aluminum.

A photoconductive layer 18 is deposited on the substrate 13 andelectrode structure 14. Suitable photoconductive materials includecadmium sulfide, cadmium selenide, alloys of cadmium sulfide or cadmiumselenide, and sensitized zinc sulfide. High-gain photoconductormaterials are presently preferred, since they provide for the largecurrents needed for heating of the magnetic material but require onlymoderate input light levels. The thickness of the layer 18 is notgenerally critical, but a layer thickness of about 3 to 10 microns ispresently preferred for resolutions of more than about ten line pairsper millimeter.

The composite recording medium 12 further includes a magnetic recordinglayer 20 which ispressed against, coated on, or deposited on thephotoconductive layer 18, to be in heat-transfer relationship therewith.Several materials and types of materials are suitable as a magneticrecording medium in the layer 20. Presently preferred materials for themagnetic recording layer 20 include low-Curie point media of the typedescribed in U.S. Pat. No. 3,176,278, by L]. Mayer, issued Mar. 30,1965, U.S. Pat. No. 3,250,636, by R.A. Wilferth, issued May 10, 1966,U.S. Pat. No. 3,368,209, by McGlauchlin et al., issued Feb. 6, 1968,U.S. Pat. No. 3,364,496, by Greiner et al., issued Jan. 16, 1968, andBritish Pat. Specification No. 1,139,232, filed Apr. 27, 1966 by E.l. duPont de Nemours and Company. Presently preferred media include MnAsfilms and ferromagnetic chromium dioxide.

In the composite recording medium 12 we prefer the use of a magneticrecording medium 20 that has an acute temperature-dependent transitionbetween the ferromagnetic and paramagnetic states at the Curie point.Where this is not inherent in the material, the preferred acuity may,according to the allowed'U.S. Pat. application, Ser. No. 649,540,Magnetic Information Recording, filed June 28, 1967, by James U. Lemke,and assigned to the subject assignee, now US. Pat. No. 3,541,577, berealized by providing the low- Curie point magnetic particles with ashape anisotropy that dominates their crystal anisotropy. v

Two preferred magnetic recording techniques includeinformation-selective thermal demagnetization and information-selectivethermoremanent magnetization. In selective demagnetization the recordingmedium 20 is typically premagnetized and is thereupon demagnetized byabove-Curie point heating until the remaining magnetized areas present arecord of the input image. In thermoremanent magnetization a thermalpattern which represents the input image is imposed upon the typicallyunmagnetized recording medium 20 so that selected portions of thatmedium are heated above Curie point. These portions are thereupon cooledin the presence of a magnetic field whereby a thermoremanentmagnetization of the type described by CD. Mee, The Physics of MagneticRecording (North-Holland Publishing Company, 1964), pp. 80-84, and inthe above mentioned Greiner et al'. U.S. Patent and British du PontPatent Specification takes place. If desired, strong positive magneticrecords may be produced in the manner disclosed in Patent application,Ser. Nos. 821,232 and 821,432, filed by Luc P. Benoit on May 2, 1969,and assigned to the subject assignee, now U.S. Pat. Nos. 3,61 1,420 and3,582,877 respectively. As will become apparent in the further course ofthis disclosure, either of these magnetic recording techniques, oracombination of these techniques to be more fully described below may beemployed for recording the magnetic image.

The apparatus further includes a source of light 22, such as anincandescent lamp or a fluorescent tube, which is energized from anelectric power source 23 and which is combined with a reflector andshade 24 that defines a narrow longitudinal slit 25 for the emission ofa sheet of light 26. The light sheet is reflected by a mirror 28 whichis tiltable about a longitudinal axis 29 and which reflects theimpinging light in the form of a light sheet 30 onto a master record 31that may, for instance, be supported on a sheet of glass (not shown).

The light 32 reflected by the master record 31 is imaged by a lens 34onto the interdigitated electrode structure 14 and photoconductive layer18.

The first and second electrodes 15 and 16 of the interdigitatedelectrode structure 14 are connected to opposite terminals of a sourceof electric energy 36. A switch 37 and a variable resistor 38 areconnected in this circuit to control timing and intensity of the currentflow to the electrode structure 14 and through the photoconductor layer18.

If the magnetic record is to be established by thermoremanentmagnetization, then the apparatus 10 requires means for magnetizingportions of the magnetic recording layer 20 that, after an image-wisethermal exposure, cool down through the Curie point of the magneticrecording medium. If the magnetic record is to be established byselective demagnetization, then the apparatus 10 requires means forpremagnetizing the magnetic recording layer 20 prior to its image-wisethermal exposure. The same elongated magnetizing head 40 may be employedfor both methods with the difference that the magnetic field provided bythe head 40 is applied to the magnetic recording layer 20 prior to imageexposure if the record is to be established by selectivedemagnetization, and is applied after thermal I exposure during thesubsequent cooling step if the record is to be established bythermoremanent magnetization. v

By way of example, it is assumed that the illustrated apparatus 10operates with a combination of the two methods by imposing apremagnetization through the agency of thermoremanent magnetization andby establishing the magnetic record by an image-wise selectivedemagnetization.

The magnetizing head 40 has an air gap 42, as well as an energizingwinding 43 which is connected to a source of electric energy 44 througha switch 45 and a 1 variable resistor 46 which serve to control thetiming and intensity of the magnetic field 48 provided at the air gap42.

To provide for a premagnetization, the photocon ductive layer 18 andinterdigitated electrode structure 14 are subjected to a uniformexposure. A flood lamp (not shown) which uniformly irradiates theelectrode structure 14 and photoconductive layer 18 may be provided forthis purpose. A light exposure which operates within a narrow exposureband 50 that extends across the, alternating electrodes 15 and 16 andthat progresses in the direction of an arrow 51 parallel to theelectrode 15 and 16 is, however, preferred for two reasons. First, ifthe exposure is at any instant limited to a narrow exposure band, thenthe intensity of electric current flow is very substantially reducedrelative to intensities that would occur if the entire composite mediumwere exposed at that instant. Secondly, it is easier to impose apremagnetization that has the requisite uniformity over the recordinglayer if such premagnetization takes place successively along aprogressing band, rather than over the entire recording layer at once.Also, the total electric current flow is distributed over the electrodesif the exposure band extends across the electrodes rather. than parallelthereto.

Accordingly, a light-reflecting element 53, only part of which has beenshown, is initially substituted for the master record 31. The reflectingelement 53, may, for

these measures.

instance, include a white sheet of paper or a mirror. The exposure band50 is then swept across the composite recording medium 20 in thedirection of the arrow 51 by a tilting of the mirror 28 about its axis29. This tilting motion is effected by a drive 54 which is coupled tothe mirror 28 and also to the magnetizing head 40 so as to move thishead in the direction of an arrow 56 and in proportion to the tiltingmotion of the mirror 28, in such mutual synchronism that the magneticfield 48, which extends across the magnetic recording layer 20, followsthe exposure band 50 in the direction of the arrow 51.

The switch 37 is closed and the variable resistor 38 is adjusted tosupply sufficient electrical energy to the interdigitated electrodestructure 14 so that successive portions of the magnetic recording layerare heated above the Curie point of the magnetic recording medium, anddo thereupon cool through the Curie point as the exposure band 50travels along the arrow 51. The position of the traveling magnetizinghead 40 relative to the moving exposure band 50 is selected such thatthe magnetic field 48, which extends in parallel to the exposure band50, trails this exposure band at such a distance that the magnetic field48 is successively present at all the magnetic layer portions that coolback through their Curie point.

In preparation of this thermoremanent magnetization, the switch 45 isclosed and the variable resistor 46 is adjusted so that the magneticfield 48 has an intensity which is insufficient to magnetize the layer20 while the same remains at a temperature below its Curie point, butwhich is sufficient to magnetize portions of the layer 20 that coolthrough their Curie point. As is known in magnetics, thermoremanentmagnetization has the highest'efficiency and linearity for all forms ofmagnetization.

The source which energizes the magnetizing head 40 may be a source ofdirect current, in which case the thermoremanent premagnetizationproduces in the layer 20 a pattern of magnetized lines that extend inparallel to the electrodes and'l6 and occupy regions mal diffusion fromadjacent regions, but a diffusion effeet can easily-be overcome bydecreasing the light intensity of. the source 22 or by increasing thetravelingspeed of the exposure region 50, or by a combination of Incontrast to a uniform premagnetization, a line-pattern premagnetizationprovides a multitude of sharp magnetic gra'dien ts which promotelarge-area fill-in, increased contrast and increased resolution.

If a point-pattern, rather than a line-pattem, is desired, then thereflecting element 53 may be provided with a pattern ofalternatinglight-reflecting and light-absorbing areas, or the source 44may be a source of current pulses, whereby every line 58 is onlymagnetized along spaced points, as shown at 60. Point-patterns aresometimes preferred toline-patterns as is well known in the book andnewspaper printing arts.

If the source 44 is a source of alternating current, each of thepremagnetization lines 58 possesses a pattern of alternatingmagnetizations as shown in FIG. 2 at 62. In practice, this correspondsto a point-pattem magnetization, since no magnetizing effect takes placeat and adjacent the zero crossovers of the alternating current. A strongtoner attraction may result in this type of pattern from the alternatingpolarity of the magnetized points.

After the premagnetization step has been completed the reflectingelement 53 is replaced by an information master record 31 that may, forinstance, be a drawing, a writing or a printed text. By way of example,it is assumed in FIG. 1, that the master record 31 is a white sheet ofpaper on which a black character 65 has been printed.

For imaging according to the embodiment shown in FIG. 1, the switch 45is opened and the magnetizing head 40 is decoupled from the drive 54 sothat no magnetic field 48 follows the exposure band 50. The mirror 28is, however, coupled to the drive 54 so that the light sheet 30 is movedacross the master record 31 whereby successive portions of the masterrecord are illuminated within a traveling narrow band 64. As before, thelens 34 images the band 64 onto the composite recording medium 12whereby the latter is exposed within an exposure band 50 that travels inthe direction of the arrow 51. The switch 37 is again closed and thevariable resistor 38 adjusted so thatelectric currents in thephotoconductive layer 18 heat the magnetic recording layer 20 above itsCurie point at locations at which light reflected by the whitebackground of the master record 31 impinges on the photoconductive layer18. This above-Curie point heating causes a demagnetization of therecording layer 20 or of its premagnetized line or point pattern. Anapplication of magnetic fields to the recording layer 20 is avoided atthis stage to preclude an undesired remagnetization of demagnetizedareas through the agency of thermoremanent magnetization.

No significant light is reflected by the black character65, wherebyportions of the photoconductive layer that correspond to that characterremain dark. Since no electric currents are caused to flow at thosecorresponding portions, the temperature of the magnetic recording layer20 remains below its Curie point within an outline occupied by an imageof the character 65. The resulting lack of demagnetization of thepremagnetized line or point-pattern leads to a magnetic record 68 of thecharacter 65 as shown in FIG. 3.

A further substantial advantage of the apparatus, methods and media ofthe subject invention may be considered at this juncture. Informationrecording methods that rely on a selective demagnetization of apremagnetized medium are frequently hampered by the fact that areaswhich have been demagnetized are remagnetized by the magneticinformation record itself -when the recording medium cools back throughits 18 located midway between adjacent electrodes and 16 tend to acquirea higher temperature than areas corresponding to points located moreclosely to the electrodes. The reason for this resides in the fact thatthe photoconductive layer portions below the electrodes 16 remain darkand do thus not produce heat-generating electric currents, and that theelectrodes themselves provide a heat-sink effect. Accordingly, thepremagnetization lines 58, 60 or 62 can easily be made narrow and spacedfrom each other whereby a thermoremanent remagnetization efiect broughtabout by the information record during cooling will limit itself to abroadeningof the premagnetization lines or points until the same havedimensions desired for a printout of the magnetic image. This broadeningis very beneficial since the thermoremanent magnetization effect underdiscussion typically provides magnetized fringes that have a polarityopposite to the magnetic polarity of the adjacent regions whereby strongmagnetic gradients provide a sharp outline of the details of themagnetic image.

The magnetic image 68 may be stored in the magnetic recording mediumand'printed out as often as desired. A magnetic toner 70 may be appliedto the magnetic image 68 to render the same visible or printable.Magnetic toners typically comprise magnetically attractable particles 72in powdered form or liquid suspension. Suitable materials for the tonerparticles 72' include iron, nickel, cobalt'or ferromagnetic alloys. Byway of example and not by way of limitation, preferred materials for thetoner particles72 include submicron particles of iron, carbonyl iron,and magnetite (Fe O Magnetic toners, toning techniques, and toningapparatus are disclosed in U.S. Pat. No. 2,932,278, by .l.C. Sims,issued Apr. 12, 1960; U.S. Pat. No. 3,052,564, by F.W. Kulesza, issuedSept. 4, 1962; and U.S. Pat. No. 3,250,636, by R.A. Wilferth, issued May10, 1966.

The toner image 74 resulting from a toning of the magnetic image 68 maybe printed out on a sheet of paper, on a film, or on another materialhaving similar surface qualities. If desired, a sheet of paper 75 may beprovided with an adhesive coating 76 to assure a transfer and retentionof the toner image 74 in the form of a copy 78 of the original character65. To effectsuch a transfer, the adhesive paper 75 is pressed onto themagnetic recording layer 20 and is thereupon removed whereby the tonerimage 74 adheres to and is pulled off together with the paper 75. As isknown in the art of magnetic printing, the toner 70may be present in theform of a magnetic ink that is absorbed by the paper 75, or the tonerparticles may be provided with shells of a thermoplastic or otherfusible material that may be fused to the paper 75 by a combination ofpressure and heat. No adhesive coating 76 is provided if a magnetic inkor a fusible material is employed.

If no further printout or storage is desired, the magnetic image 68 maybe erased by such conventional methods as an exposure of the magneticrecording layer 20 to anhysteretic magnetic erasing fields, or toabove-Curie point heating. The latter may be efiected by a sweeping ofthe light sheet 32 across the interdigitated electrode structure 14 andphotoconductive layer 18, while the electrodes are connected to theelectric energy source 36 and the reflective element 53 is substitutedfor the information record 31.

tact with, and removable from, the photoconductive layer 18.

Reverting to the interdigitated electrode structure 14 it should benoted that the same affords a much better management of current supplythan systems in which the photoconductive layer is sandwiched betweensheet electrodes. In the interdigitated structure shown in FIG. 1, anyexposed point of the photoconductive layer is serviced by two currentsupply electrodes. If a light exposure shifts in the direction of thearrow 51, then an increase in electrical resistance along an electrode15 is automatically compensated by a corresponding decrease inelectrical resistance along an adjacent electrode l6. Electrode sheets,on the other hand, do not provide a similar favorable rheology so thatcontrast, resolution and quality of image points tend to be dependent onthe location of the particular point relative to the edges of therecording medium.

We claim:

comprising in combination the steps of:

providing a plurality of interdigitated first an second electrodes;

providing a photoconductive layer in electrical contact with andcontiguous to said interdigitated first and second electrodes; providingin heat-transfer relationship with and contiguous to saidphotoconductive layer a magnetic image recording layer susceptible to animage-wise change of magnetization in response to a thermal imagepattern; connecting said interdigitated first and second electrodes toopposite terminals, respectively, of a source of electric energy;applying electrical energy from said source to said interdigitated firstand second electrodes by way of said opposite terminals and exposingsaid photoconductive layer to said image to produce a thermal patterncorresponding to said image and constituting said thermal image pattern;and magnetically recording said image by changing the state ofmagnetization of said recording layer in response to said thermalpattern. 2. A method as claimed in claim 1, wherein: saidphotoconductive layer is provided with a first side located at saidplurality of interdigitated first and second electrodes, and with asecond side opposite said first side; and 1 said magnetic recordinglayer is provided in heattransfer relationship with said photoconductiveto said pattern so as to demagnetize said portions of said recordinglayer. 4. A method as claimed in claim 3, wherein:

said recording layer is premagnetized by cooling said recording layerthrough its Curie temperature in the presence of a magnetic field.

5. A method as claimed in claim 3, wherein:

said recording layer is premagnetized by thermoremanent magnetizationincluding a heating of said recording layer with the assistance of anelectrical energization and a progressive exposure of saidphotoconductive layer in a direction progressing in parallel to saidelectrodes, and a subsequent cooling of said recording layer in thepresence of a magnetic field.

6. A method as claimed in claim 1, wherein:

said photoconductive layer is progressively exposed to said image in adirection progressing in parallel to said interdigitated electrodes.

7. A method as claimed in claim 1, wherein:

said photoconductive layer is exposed to said image within a band ofexposure extending substantially across said interdigitated first andsecond electrodes and progressing in a direction parallel to saidinterdigitated electrodes.

8. A method of magnetically recording an image on a composite recordingmedium, which medium includes a photoconductive layer, a plurality ofinterdigitated first and second electrodes in electrical contact withand contiguous to said photoconductive layer, and, in heat-transferrelationship with and contiguous to said photoconductive layer, amagnetic image recording layer susceptible to an image-wise change ofmagnetization in response to a thermal image pattern, comprising incombination the steps of:

connecting said interdigitated first and second electrodes to oppositeterminals, respectively, of a source of electric energy;

applying electrical energy from said source to said interdigitated firstand second electrodes by way of said opposite terminals andprogressively exposing said photoconductive layer to said image in .adirection progressing parallel to said interdigitated electrodes toproduce a thermal pattern corresponding to said image and constitutingsaid thermal image pattern; and

magnetically recording said image by changing the state of magnetizationof said recording layer in response to said thermal pattern.

9. A method as claimed in claim 8, wherein:

said photoconductive layer is exposed to said image within a band ofexposure extending substantially across said interdigitated first andsecond electrodes and progressing in a direction parallel to saidinterdigitated electrodes. 10. A method as claimed in claim 8, wherein:

the state of magnetization of said recording layer is changed bypremagnetizing said recording layer and heating with said thermalpattern portions of said premagnetized recording layer corresponding tosaid pattern so as to demagnetize said portions of said recording layer.

11. A method as claimed in claim 10, wherein:

said recording layer is premagnetized by cooling said recording layerthrough its Curie temperature in the presence of a magnetic field.

12. A method as claimed in claim 10, wherein:

said recording layer is premagnetized by thermoremanent agnetiza ioincluding a heatin of said recording ayer wit t e assistance of an eectrical energization and a progressive exposure of said photoconductivelayer in a direction progressing in parallel to said electrodes, and asubsequent cooling of said recording layer in the presence of a magneticfield.

13. A composite photoelectric recording medium operable with a source ofelectric energy having opposite terminals, comprising in combination:

a thermally responsive magnetic image recording layer contiguous to saidphotoconductive layer. 14. A composite recording medium as claimed inclaim 13, including:

a transparent substrate carrying said interdigitated first and secondelectrodes and said photoconductive layer.

15. A composite recording medium as claimed in claim 13, wherein:

said photoconductive layer has a first side located at said plurality ofinterdigitated first and second electrodes, and a second side oppositesaid first side; and

said magnetic recording layer is in heat-transfer relationship with saidphotoconductive layer through said second side of said photoconductivelayer.

U l 0 i -+zg g STATQESYPATENT OFFICE CERTIFICATE OF CORRECTION I PatentNo. 3,717, 460 Dated 973 Duck Frederick J.Je r

Inventofls) Sherman W.

Ir 1s certified that error appears in the above-identified patent 1 andthat: said Letters Patent are hereby corrected as shown below:

!" Column 2, line 19, in heat-tiafisfer relationship with and. should.be inserted after "and".- f v Column 3, line 5, insert comma. after"electrodes" Signed and sealed this 26th dy of November 1974.

I (SEAL) -Attest: M coY M. GIBS ON'JR. c. MARSHALL DAN-N A ttestingfiffieer Y 7 Commissioner of Patents mg UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3 ,717,460 Dated 2 973 Inventofls)Sherman W.Duck, Frederick JJefi'egg, lgmgg n l gmkg It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

I'" Column 2, line 19, in heat-trazisfer relatibnship with and should beinserted after "and". v Column 3, line 5, insert comma. after"electrodes" Signed and sealed this 26th dy of Ndvefnber 1974.

(SEAL) Attest: v

McoY M. snsson JR. 0. MARSHALL DAN-N Attesting Officer Commissioner ofPatents

1. A method of magnetically recording an image, comprising incombination the steps of: providing a plurality of interdigitated firstand second electrodes; providing a photoconductive layer in electricalcontact with and contiguous to said interdigitated first and secondelectrodes; providing in heat-transfer relationship with and contiguousto said photoconductive layer a magnetic image recording layersusceptible to an image-wise change of magnetization in response to athermal image pattern; connecting said interdigitated first and secondelectrodes to opposite terminals, respectively, of a source of electricenergy; applying electrical energy from said source to saidinterdigitated first and second electrodes by way of said oppositeterminals and exposing said photoconductive layer to said image toproduce a thermal pattern corresponding to said image and constitutingsaid thermal image pattern; and magnetically recording said image bychanging the state of magnetization of said recording layer in responseto said thermal pattern.
 2. A method as claimed in claim 1, wherein:said photoconductive layer is provided with a first side located at saidplurality of interdigitated first and second electrodes, and with asecond side opposite said first side; and said magnetic recording layeris provided in heat-transfer relationship with said photoconductivelayer through said second side of said photoconductive layer.
 3. Amethod as claimed in claim 1, wherein: the state of magnetization ofsaid recording layer is changed by premagnetizing said recording layerand heating with said thermal pattern portions of said premagnetizedrecording layer corresponding to said pattern so as to demagnetize saidportions of said recording layer.
 4. A method as claimed in claim 3,wherein: said recording layer is premagnetized by cooling said recordinglayer through its Curie temperature in the presence of a magnetic field.5. A method as claimed in claim 3, wherein: said recording layer ispremagnetized by thermoremanent magnetization including a heating ofsaid recording layer with the assistance of an electrical energizationand a progressive exposure of said photoconductive layer in a directionprogressing in parallel to said electrodes, and a subsequent cooling ofsaid recording layer in the presence of a magnetic field.
 6. A method asclaimed in claim 1, wherein: said photoconductive layer is progressivelyexposed to said image in a direction progressing in parallel to saidinterdigitated electrodes.
 7. A method as claimed in claim 1, wherein:said photoconductive layer is exposed to said image within a band ofexposure extending substantially across said interdigitated first andsecond electrodes and progressing in a direction parallel to saidinterdigitated electrodes.
 8. A method of magnetically recording animage on a composite recording medium, which medium includes aphotoconductive layer, a plurality of interdigitated first and secondelectrodes in electrical contact with and contiguous to saidphotoconductive layer, and, in heat-transfer relationship with andcontiguous to said photoconductive layer, a magnetic image recordinglayer susceptible to an image-wise change of magnetization in responseto a thermal Image pattern, comprising in combination the steps of:connecting said interdigitated first and second electrodes to oppositeterminals, respectively, of a source of electric energy; applyingelectrical energy from said source to said interdigitated first andsecond electrodes by way of said opposite terminals and progressivelyexposing said photoconductive layer to said image in a directionprogressing parallel to said interdigitated electrodes to produce athermal pattern corresponding to said image and constituting saidthermal image pattern; and magnetically recording said image by changingthe state of magnetization of said recording layer in response to saidthermal pattern.
 9. A method as claimed in claim 8, wherein: saidphotoconductive layer is exposed to said image within a band of exposureextending substantially across said interdigitated first and secondelectrodes and progressing in a direction parallel to saidinterdigitated electrodes.
 10. A method as claimed in claim 8, wherein:the state of magnetization of said recording layer is changed bypremagnetizing said recording layer and heating with said thermalpattern portions of said premagnetized recording layer corresponding tosaid pattern so as to demagnetize said portions of said recording layer.11. A method as claimed in claim 10, wherein: said recording layer ispremagnetized by cooling said recording layer through its Curietemperature in the presence of a magnetic field.
 12. A method as claimedin claim 10, wherein: said recording layer is premagnetized bythermoremanent magnetization including a heating of said recording layerwith the assistance of an electrical energization and a progressiveexposure of said photoconductive layer in a direction progressing inparallel to said electrodes, and a subsequent cooling of said recordinglayer in the presence of a magnetic field.
 13. A composite photoelectricrecording medium operable with a source of electric energy havingopposite terminals, comprising in combination: a plurality ofinterdigitated first and second electrodes; said interdigitated firstand second electrodes connected to opposite terminals, respectively, ofsaid source of electric energy; a photoconductive layer in electricalcontact with and contiguous to said interdigitated first and secondelectrodes; and a thermally responsive magnetic image recording layercontiguous to said photoconductive layer.
 14. A composite recordingmedium as claimed in claim 13, including: a transparent substratecarrying said interdigitated first and second electrodes and saidphotoconductive layer.